1. Field of the Invention
The present invention relates generally to an actively controlled suspension control system which varies suspension characteristics depending upon the vehicle driving condition. More specifically, the invention relates to an actively controlled suspension system which controls line pressure in the suspension system even when the pressure in a fluid source is substantially low or zero.
2. Description of the Background Art
The Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 51-42215 discloses a hydropneumatic suspension system with vehicle height control operation. The disclosed system employs a fluid pump driven by a vehicular engine and an hydraulic cylinder. Pressurized fluid from the fluid pump is introduced into the hydraulic cylinder for adjusting the fluid pressure in the hydraulic cylinder for performing height adjustment of the vehicle. A poppet valve is employed for establishing fluid pressure from the fluid pump to the hydraulic cylinder and blocking fluid communication therebetween. When the engine stops, the fluid pump is accordingly stopped. The shown system allows manual operation for locking the line pressure by operating the poppet valve. Therefore, in the construction disclosed in the Tokkai Showa No. 51-42215, the line pressure in the hydraulic circuit of the suspension system can be maintained at the pressure immediately before the poppet valve is shut.
The poppet valve is thus effective in locking the line pressure in the hydraulic circuit. On the other hand, the poppet valve encounters difficulty in controlling fluid pressure in the hydraulic circuit in pressure feedback manner, since it merely operates in ON/OFF manner.
In the actively controlled suspension, it is required to control the pressure to be introduced into an hydraulic cylinder depending upon relative displacement between a vehicle body and a wheel axle. Therefore, as illustrated in the Japanese Patent First Publication (Tokkai) Showa No. 61-213911, a spool valve is employed for adjusting the pressure in the hydraulic cylinder to adjust the suspension characteristics. Control of the position of the spool valve is performed utilizing fluid pressure in one of a pair of pressure chambers in the hydraulic cylinder which has higher pressure than the other, as a pilot pressure. Namely, the spool valve adjusts path area in a path of working fluid depending on the magnitude of the pilot pressure so as to adjust the pressure in the pressure chambers and whereby adjusts the suspension characteristics in response to relative motion of the vehicle body and wheel axle.
In case of the spool valve, it is inevitable to cause internal leakage of the working fluid through a gap between the spool valve and a valve guide. Especially, magnitude of leakage becomes substantial when the pressure difference across the spool valve is great. Therefore, when the fluid pump is inoperative and thus the source pressure is substantially low, the significant pressure difference is created between the hydraulic cylinder and the fluid pump to cause leakage of the pressure from the hydraulic cylinder to the fluid pump or the fluid source. Therefore, while engine is not driven, pressure in the hydraulic cylinder drops at substantial level.